One common technique used in wireless communication is code division multiple access (CDMA) signal modulation in which multiple communications are simultaneously conducted over a radio-frequency (RF) spectrum. Some example wireless communication devices that have incorporated CDMA technology include cellular radiotelephones, PCMCIA cards incorporated within computers, personal digital assistants (PDAs) equipped with wireless communication capabilities, and the like.
A conventional architecture for a CDMA receiver includes a radio-frequency (RF) section and an infrared (IF) section. In particular, the received RF signals are typically filtered in the RF section, converted from RF signals to IF signals for further filtering and scaling by a voltage gain amplifier (VGA) in the IF section, and finally converted to baseband signals. The baseband signals are typically passed through an analog-to-digital (A/D) converter to produce digital samples which can be sent to a digital signal processor for tracking and demodulation.
The Zero infrared frequency (Zero IF) architecture is a more recent architecture used in CDMA wireless communication devices. Unlike other conventional architectures, the Zero IF architecture converts incoming RF signals directly into baseband signals without first converting the RF signals to IF signals. In particular, the Zero IF architecture makes use of a digital VGA that scales the digital samples produced by the A/D converter. In this manner, the Zero IF architecture eliminates the need for various IF components, including an IF mixer, an IF VGA and IF filters.
In the heterodyne architecture with an IF section, the IF-VGA controlled by an automatic gain control unit (AGC) is responsible for either expanding or compressing the signal such that it fits in the relatively narrow dynamic range of the A/D converter. The A/D converter can then produce small bit-width (typically 4 bits) numbers so that rest of the hardware that performs signal processing can be simplified. In the Zero-IF architecture however, due to the absence of the IF VGA, the A/D converter is typically designed to have much larger dynamic range resulting in large bit-width numbers at the output.
Although the Zero IF architecture eliminates the need for IF components, the architecture may require more complicated baseband components, primarily due to the relatively large digital signals (typically 18 bits) generated by the A/D converter. Consequently, a digital VGA is implemented at baseband to scale the large bit-width signals from the A/D converter. The Zero IF architecture may implement a relatively wide multiplier (typically an 18-bit by 18-bit multiplier) to scale the large digital signals. In addition, the digital VGA typically includes a relatively large lookup table (LUT) (often exceeding a kilobyte or more) to convert values received from the AGC unit from logarithmic units in decibels (dB) to linear values for controlling the gain of the digital VGA. In operation, for example, the digital VGA multiplies the linear digital signals received from the A/D converter by a linear gain value obtained from the LUT. For these reasons, wireless communication devices incorporating the Zero IF architecture may have significant cost even though the IF components have been eliminated.
The amount of memory space needed to store one or more lookup tables is generally proportionate to the amount of information stored within the lookup tables. Thus, as lookup tables become larger, the use of lookup tables can become memory intensive. For some wireless communication devices, memory space can be quite limited, thus making lookup tables difficult and/or costly to implement.